1. Field of the Invention
The present invention relates to a method for treating heater drain water in a power plant.
2. Description of the Related Art
Typically, fine particles of iron oxides are present in heater drain water generated at pressurized water reactor nuclear power plants (hereinafter abbreviated simply as xe2x80x9cPWRxe2x80x9d), in boiling water reactor nuclear power plants (hereinafter abbreviated simply as xe2x80x9cBWRxe2x80x9d), or in fossil-fueled power plants. These fine particles of iron oxides must be removed.
For example, in a PWR, impurities such as metal oxides mainly consisting of fine particles of iron oxides are introduced along with the feed water to devices on the secondary side of a steam generator (hereinafter abbreviated simply as xe2x80x9cSGxe2x80x9d) which produces steam for driving a turbine. While power is generated, these impurities gradually adheres to the outer surfaces of heat transfer pipes or a clevis section of the SG, causing reduction in heat transfer efficiency and corrosion of the heat transfer pipes. Because of this, conventionally, various methods have been attempted for reducing the concentration of impurities within the feed water which are transferred to the secondary side of SG.
Similar to the case of a PWR as described above, in a thermal power plant, impurities such as described above contained in the feed water to a boiler gradually adheres onto the outer surfaces of heat transfer pipes of the boiler, and the differential pressure through the boiler is increased by the adhered substances. Because of this, conventionally, various methods have been attempted for reducing the concentration of impurities within the feed water.
The impurities having fine particles of iron oxides as the main constituent and contained in the feed water to the SG or to the boiler can be mainly divided into two portions, a portion introduced from the condensate line and a portion introduced from various heater drain systems (a low pressure feed water heater drain, a moisture separator drain, a high pressure feed water heater drain and so forth). In the condensate line, a condensate treatment system is provided which comprises a condensate pre-filter and a condensate demineralizer or a condensate demineralizer alone, and therefore introduction of impurities from the condensate line is relatively small. Therefore, in order to reduce the concentration of impurities in the feed water to the SG and to the boiler, it would be effective to reduce particularly the amount of impurities introduced from various heater drain systems downstream of the condensate treatment system.
The forms of iron impurities which are the primary constituents of the impurities in heater drain water systems in power plants will now be described. In a heater drain water system, in order to prevent corrosion of the equipment, pipes, etc., deoxidizing treatment, and pH adjusting through an addition of ammonia or various amines such as ethanolamine are performed so that the concentration of dissolved oxygen in the water is maintained at a few ppb or less and the pH of the water is maintained at a value of approximately 9. The pH adjustment is made in order to inhibit elution of iron ions from pipes or the like, and therefore, the percentage of ionic iron in the heater drain water is low and most of the iron to be removed is in the form of fine particles of iron oxides. Even under such conditions of water quality, due to corrosion of equipment, pipes, etc., a thin film of triiron tetroxide (Fe3O4; more commonly referred to as xe2x80x9cmagnetitexe2x80x9d) is formed on the surfaces of the equipment, pipes, etc. This film is steadily chipped away by the generated steam, resulting in production of fine particles of iron oxides. This phenomenon is commonly referred to as xe2x80x9cerosionxe2x80x9d. It can be presumed that because these phenomena of formation of thin film of triiron tetroxide and steady chipping away of the formed thin film by the steam continue, very fine particles of iron oxides that are finer than the iron oxide particles within the condensate are formed and contained within the heater drain water.
The fine particles of iron oxides in the heater drain water exist in a form of vary fine particles due to the erosion phenomenon described above, and it has been known that removal rate of these particles using a filter or the like having a commonly used filter rating (pore size) is not stable. Moreover, the use of an electromagnetic filter or a magnetic filter for removing the impurities having such fine particles of iron oxides as the main constituent from the heater drain water also results in similar unstable removal rates for impurities, and thus, has not been put into practice.
As a solution to this problem, Japanese Patent Laid-Open Publication No. Hei 11-57416 proposes a method wherein oxygen is mixed into the heater drain water of power plant before the heater drain water reaches the filter so that the fine particles of iron oxides can be converted into a form that can be easily captured by the filter. According to this method, by injecting oxygen into the heater drain water so that the concentration of dissolved oxygen in the heater drain water after the mixing treatment is in a range of 20 ppb-100 ppb (=xcexcg/L (liter; also apply similarly to the following descriptions)), and more preferably, in a range from 40 xcexcg/L-100 xcexcg/L, it is possible to achieve a removal rate of 90% or greater for the impurities by the filter.
However, for the feed water to the SG or boiler, it is desired that the concentration of dissolved oxygen be maintained below 5 xcexcg/L-7 xcexcg/L in view of prevention of corrosion or the like, and, therefore, dissolving oxygen to a concentration exceeding this range is not desirable.
The present invention was conceived to solve the above described problem and an object of the present invention is to provide a method for treating heater drain water in a power plant which can stably remove fine particles of iron oxides contained in heater drain water in a power plant.
Through extensive studies of methods for removing iron constituents in heater drain water of power plant, the present inventors have found that by adjusting the amount of oxygen to be injected to a range which results in a very small concentration within a range from 1 ppb to 20 ppb of dissolved oxygen in the heater drain water after oxygen mixing, the impurity removal capability of the filter can be drastically improved and made the present invention based on the finding.
According to one aspect of the present invention, in a process of removing fine particles of iron oxides from heater drain water of power plant containing such fine particles of iron oxides through a solid-liquid separation operation using a filter, oxygen is mixed into the heater drain water of power plant before the heater drain water reaches the filter so that the concentration of dissolved oxygen in the heater drain water after the oxygen mixing is in a range from 1 xcexcg/L to 20 xcexcg/L, and more preferably, in a range from 1 xcexcg/L to 5 xcexcg/L. With such treatment, the fine particles of iron oxides can be transformed into a form that can be readily captured and removed using the filter.